One of conventionally known position determination methods is a method using GPS satellites possessed by the United States. This method using GPS is one of the satellite navigation methods. That is, based on signals received from multiple satellites whose positions are known, the distances between the self and the satellites are computed; and the present position of the self is computed from the computed distances and the positions of the satellites. Hereafter, satellites for satellite navigation are satellites for use in satellite navigation. A receiver for satellite navigation or a satellite navigatin receiver is a device that receives signals from satellites for satellite navigation and computes its own position. An antenna for satellite navigation or a satellite navigation antenna is an antenna for a receiver for satellite navigation to receive signals from satellites for satellite navigation.
In methods using GPS satellites, radio waves are received from three or more GPS satellites, and the distances from the individual satellites are computed. Then the present position is determined from the resultant distance data. The position determination accuracy of these methods is 10 to 30 m or so. To enhance the position determination accuracy, the following are considered: use of the GPS L2-band (1227.6 MHz) and L5-band (1176.45 MHz), launching of new satellites for satellite navigation, including Galileo and quasi-zenith satellites, introduction of RTK-GPS technology using an electronic reference point, and the like. As a result, there is a possibility that the position determination accuracy of 2 to 3 cm or less can be attained.
Use of the above-mentioned technologies makes it possible to provide a receiver for satellite navigation that can execute both the conventional position determination with low accuracy of 10 to 30 m or so and position determination with high accuracy of 2 to 3 cm or so. With such a receiver for satellite navigation, the results of position determination with multiple accuracies can be selectively used according to mode of utilization and usage.
After due consideration, however, the inventors concluded that position determination based on such satellite navigation involved a problem of increased power consumption.
An example will be taken. It will be assumed that signals whose chip rate is high are decoded (equivalent to de-spreading) to execute high-accuracy position determination. (Chip rate is defined as the amount of data per unit time of chips for spreading and de-spreading signals transmitted from satellites.) Constant decoding at a high chip rate in this case increases the processing load and the power consumption of equipment that decodes.
Further, it will be assumed that a receiver for satellite navigation is provided with reception circuits in multiple systems to execute position determination with multiple accuracies. Here, the reception systems are capable of receiving signals from satellites for satellite navigation of multiple frequencies. When the reception circuits in multiple systems are constantly operated regardless of whether they are required or not, power supply to unused reception circuits increases the power consumption.